It can be appreciated that wood strength grading has been in use for many years. This has traditionally been accomplished by using visual grading rules to predict strength. Other technologies such as mechanical bending and X-ray, to sense density, have been used to estimate the strength of wood.
The main problem with conventional visual wood grading is that is does not predict strength or stiffness accurately. The use of the mechanical bending improved the ability to predict stiffness of the lumber but the correlation to strength is poor. X-ray based systems predict strength and stiffness based on density only.
While these devices have been suitable for the particular purpose to which they addressed, they are not as suitable for highly accurate strength and stiffness prediction of today's variable and often low-quality wood resource.